1. Field of the Invention
The present invention relates to a display device with LEDs (Light Emitting Diodes), and more particularly to a display device of a type in which LEDs are surrounded by a reflection case of a predetermined pattern to obtain a light emitting shape of that pattern by means of lightening of the LEDs. This display device with LEDs is, e.g. as a numeric and symbolic display device of a segment type, applied to a counter of a measuring instrument or a mechanical apparatus, or to display various physical quantities such as time.
2. Description of the Background Art
In conventional display devices with LEDs, there is a numeric and symbolic display device of a segment type as shown in FIG. 1. Such a display device, in which LEDs are attached in the respective grooves of a reflection case 1 having a plurality of reflection structures of a groove shape corresponding to the respective segments, displays desired figures by lightening predetermined LEDs according to a driving signal externally supplied through a lead 2.
Such conventional display devices with LEDs are generally divided into the following two types according to difference the of their structures.
One type of display device, referred to as a substrate type, has a sectional structure shown in FIG. 2. Referring to FIG. 2, in this display device, an LED 13 is mounted on an electrically interconnected substrate 14, and a reflecting case 11 having an upper surface on which a dispersion sheet 15 is attached is provided on the substrate 14. The LED 13 is fixed on the substrate 14 with a silver paste, electrically connected to the electric interconnection on the substrate 14 by means of bonding by a gold wire 16, and also electrically connected to outside portions by a pin 12 soldered to the substrate 14.
Another type of display device is referred to as a molded type, of which the sectional structure is shown in FIG. 3. In this display device, referring to FIG. 3, an LED 23 is mounted to one end of a formed lead frame 22, and the lead frame 22 and the LED 23 are connected with each other by means of bonding by a gold wire 26. The device has a structure in which the lead frame 22 provided with the LED 23 is inserted in a reflection case 21 having a groove-shaped reflection structure simultaneous with sealing by epoxy resin 24, and then the epoxy resin 24 is hardened. The LED 23 is electrically connected to outside portions through the respective lead terminals of the lead frame 22.
Among these conventional display devices, in the substrate type shown in FIG. 2, troubles arise such as positioning errors in providing the dispersion sheet 15, a defect that adhesive exudes outside the dispersion sheet 15, change in colors of the dispersion sheet 15 due to exposure to heat or solvents in the manufacturing process, and a defect due to dust stuck on the dispersion sheet 15. Accordingly, problems such as a decrease in quality and workability or an increase in cost arise. Furthermore, there is also a problem of moisture resistance because the device cannot be subjected to resin molding.
In the molded type shown in FIG. 3, there is no such problems as described above because the device does not use any dispersion sheet, and besides, the number of parts is decreased to enhance workability of assembling. However, since coefficients of thermal expansion of the lead frame 12 and the epoxy resin 14 are considerably different from each other, thermal stresses are likely to be produced in the resin. Therefore, defects in quality are developed that the LED is not lit because of cracks of the LED chip due to stresses, or because of disconnecting of a gold wire with a diameter of 25.mu.m-30.mu.m connecting a bonding pad (not shown) provided on the upper surface of the LED chip and the pad (not shown) on a lead frame. Furthermore, because of resin exfoliation between the outer peripheral surface of the LED chip and the resin layer surrounding it, there is an air layer between the LED chip and the resin layer. As a result, the light intensity of the LED decreases because of refraction at the boundary. Accordingly, troubles in quality are not avoided. Besides, having a large number of work steps in manufacturing, the device has a problem of high cost due to a decrease in workability.